The mouth of this segmented invertebrate has been a puzzle for many researchers.

I have a soft spot for Anomalocaris. Of all the Cambrian oddballs, this segmented invertebrate was one of the strangest. The prehistoric creature’s stalked eyes, swim flaps, spiked grasping appendages, and plated mouth gave this predator an exceptionally alien appearance. Anomalocaris was so weird, in fact, that paleontologists only recently assembled a complete picture of what this animal looked like. For years, the various parts of Anomalocaris were believed to be parts of various other creatures. What was eventually recognized as the neo-predator’s mouth was especially perplexing.

The mouth of Anomalocaris started off as a jellyfish. That’s a historical quirk, not a biological one. A century ago, as he was cataloging 505-million-year-old fossils from the exceptional Burgess Shale site, paleontologist Charles Doolittle Walcott puzzled over what seemed to be a flattened ring. Walcott called the animal Peytoia, and suggested that it was a strange form of archaic jellyfish.

Walcott’s interpretation stayed in place for decades. When paleontologists Harry Whittington and Simon Conway Morris wrote an article about Cambrian life for Scientific American in 1979, they included the odd jelly in a reconstructed Burgess Shale habitat. As Stephen Jay Gould later commented in his book Wonderful Life, the artistically-reinvigorated Peytoia looked like “a kind of Frisbee cum flying saucer cum pineapple slice.”

By 1985, however, those squishy discs were redeployed as the trilobite-crushing mouthparts of Anomalocaris and kin. As Whittington, Conway Morris, and additional collaborator Derek Briggs cleaned and studied various Cambrian fossils, they found that Peytoia—as Walcott envisioned it—was not a discrete creature, after all. Rare slabs preserved the rings on specimens of Anomalocaris and a similar, closely-related creature that was previously thought to be a sponge. These more complete specimens placed the circular mouth in context with the creature’s grasping appendages, eye stalks, swim fins, and other weird parts. Where strange prehistoric Frisbees once flew, there were now at least two bizarre Cambrian apex predators unlike anything paleontologists had seen before.

The mouth of Anomalocaris looked like a camera-shutter of doom. I loved that imagery. Surely Anomalocaris nabbed unsuspecting trilobites with its flexible arms and fed the little arthropods directly into the crushing plates which constituted its mouth. And rare fossil feces—filled with trilobite bits—throw support to the idea that some anomalocaridids were munching on hard-shelled prey.

But the notion that anomalocaridids were specialized trilobite hunters is probably too simple, and has often been questioned. Two years ago, a GSA presentation about Anomalocaris stirred up a good deal of media attention when paleontologist James Whitey Hagadorn suggested that the Cambrian predator was physically incapable of eating hard-shelled prey. Maybe Anomalocaris fed on soft trilobites that had freshly molted, but, for the most part, the nightmarish predator was probably more of a threat to worms. (As far as I’m aware, Hagadorn’s study has not yet been published, and I haven’t seen any of the various presentations on the research delivered over the past few years.)

Indeed, as scary as the mouthparts of Anomalocaris look, I might have to shelve the idea that the creature rapaciously gobbled up trilobites. As explained by paleontologists Allison Daley and Jan Bergström in a new Naturwissenschaften paper, the mouth of Anomalocaris may have been better for sucking than pulverizing.

The mouths of Peytoia (A), Hurdia (B), and Anomalocaris canadensis (C). Notice that the mouth of Anomalocaris has a triradial, rather than tetraradial, organization.

From Daly and Bergström, 2012.

As far as we know, there were three different anomalocaridids in the Burgess Shale fauna—Anomalocaris, Peytoia (commonly called Laggania*), and an even-weirder form with a pointy carapace called Hurdia. All three were believed to have very similar mouths. The standard arrangement, paleontologists thought, involved an "oral cone" made up of 32 plates, with four of those plates situated ninety degrees apart from each other. The other, spiny plates were distributed between the primary four.

*(The details are in the paper, but Daly and Bergström point out that the proper name for the animal Laggania is actually Peytoia. Even though the animal isn’t the jellyfish Walcott imagined, the name he coined in 1911 is the proper one.)

But when Daly and Bergström reexamined the mouths of Anomalocaris canadensis from the Burgess Shale, they found a different arrangement. This famous animal had only three large plates—arranged in a triangle—with a variable number of smaller plates between them. "This highlights a serious misunderstanding of one of the most renowned anomalocaridids," Daly and Bergström wrote, and this new look at the creature’s oral cone shows that Anomalocaris probably wasn’t a trilobite-crushing terror. "The central opening of Anomalocaris oral cones has an irregular shape and small size," Daly and Bergström explained, "making it unsuitable for strong biting motions." The stiff, small, circular mouth of Anomalocaris looks better suited to sucking in small prey—perhaps the animal sifted through the Cambrian mud in search of worms and other tidbits.

Not all anomalocaridids had identical mouthparts, though. Peytoia and Hurdia had the classic four-part mouth shape, and Hurdia, in particular, possessed an extra array of small spines in the middle of its mouth. Clearly these animals were feeding on disparate prey and doing so in different ways—a conclusion supported by the variations in the spiny frontage appendages anomalocaridids used to grasp prey. "As opposed to being highly specialized trilobite predators, anomalocaridids were generalists occupying a range of ecological habits, from freeswimming ambush predators to sediment-sifting scavengers," Daly and Bergström concluded. We’re only just starting to understand the ecology and biology of these absolutely fantastic animals. In this case, strange mouths hint that the celebrated idea of an intense Cambrian arms race between shutter-mouthed predators and trilobites was not as intense or dramatic as we thought.